1. Field of the Invention
One aspect of the present invention relates to a selective catalytic reduction catalyst system for lean NOx control with a broad temperature window for lean-burn gasoline and diesel engines. Another aspect of the present invention relates to a selective catalytic reduction catalyst system that provides high lean NOx conversion over a broad range of NO concentrations.
In another embodiment of this invention, a selective catalytic reduction catalyst provides effective lean NOx control with a broad temperature window, even for cold start NOx emissions.
2. Background Art
Selective Catalytic Reduction (SCR) with NH3 is a leading candidate for NOx control on lean-burn gasoline engines and diesel engines. The SCR catalyst uses base metals to promote the reaction between NOx and NH3 to produce N2 under lean conditions. The other leading candidate for NOx control on lean applications is the lean NOx trap (LNT), a catalyst containing precious metal (e.g., platinum, palladium, rhodium) which stores the NOx under lean conditions and reduces the stored NOx to N2 during periodic rich purges. Compared to a LNT, SCR offers several advantages. SCR catalysts can operate over a broader temperature range than LNTs, and SCRs do not require rich purges, which benefits the fuel economy. SCR catalysts are also more robust to the feedgas NOx concentration. Finally, SCR catalysts do not use precious metals, and thus the catalyst costs are lower.
While SCR catalysts have several advantages, SCR catalysts have certain limitations. The SCR formulations that are leading contenders for NOx control on vehicles have applicability either at the high temperature end or the low temperature end. There is thus a need for a SCR catalyst system that is effective across a broader range of temperatures to provide effective lean NOx control during both low load and high load operation on a vehicle. A broader temperature window also offers more flexibility in regards to the location of the SCR converter in the exhaust system on a vehicle. The SCR catalyst system must also provide good NOx control following a cold start on a vehicle. This is particularly true on a diesel engine, where the SCR catalyst provides the only NOx control. In addition, since the engine can emit different levels of NO, it is desirable that the SCR catalyst system provide high NOx conversion over a broad range of NO concentrations.